Nickel base alloys containing boron to lower the melting point are widely used to braze ferrous and nickel base superalloys in the temperature range 970.degree.-1200.degree. C. These alloys, which include particularly the family of alloys specified by the society of automotive engineers in aeronautical materials specifications(AMS) 4775-4779, and many more or less proprietary varients of these alloys, generally contain silicon in addition to boron, as a melting temperature depressant, and often include chromium, iron and other elements. These alloys contain metallic borides and silicides which render them extremely hard and brittle in the cast state.
Certain brazing alloys such as AMS 4779 (consisting principally of nickel, silicon and boron) described in Aerospace Material Specifications published June 30, 1960 by Society of Automatic Engineers, Inc. are extremely hard and brittle and therefore cannot readily be cold worked into suitable form, such as wire or strip, for placement in a metal joint to be brazed. In the past, such brazing alloys have been formed by grinding or atomizing the elements into a powder and mixing the powder with a suitable organic binder which holds the powder granules together in strip or wire from convenient for handling and placement. The disadvantage of brazing alloys in this form is that the organic binder on heating decomposes and gives off gases which, if evolved too rapidly, can displace or "blow away" some of the metallic brazing alloy powder. Since the brazing operation is commonly performed in hydrogen or argon or in a vacuum, the decomposition of the organic binder is seldom if ever complete and as a consequence a carbonaceous residue is left in the braze joint. This residue can inhibit the flow of the molten brazing alloy, resulting in an incompletely brazed or filled joint. A further disadvantage is that the organic-bonded powder form of alloy undergoes a reduction in volume during the brazing process which renders such alloys unsuitable for applications requiring precise prepositioning of the parts to be joined. Thus even alloy AMS 4779, which has the lowest boron and silicon content of this family of alloys, cannot practicably be rolled, hot or cold, to form foil.
In brazing applications, these alloys are widely applied in the form of a fine powder suspended in a viscous vehicle to form a paste. Metering of paste to give a precise, desired quantity of alloy at the brazing site is not easy. Generally, an excess of paste is used, over the minimum alloy quantity needed to form the braze. Besides additional cost, this excess may cause erosion problems or may result in flow of the brazing alloy to areas where its presence is undesirable. Moreover, the past vehicle may contaminate the braze alloy with undesirable impurities. It would often be preferred to use a brazing alloy preform blanked or etched from foil. However, present foil making processes are costly, and economics are presently weighted in favor of the use of powdered alloys.
It is known that when alloys of this class are rapidly solidified from the liquid state, the formation of embrittling compounds can be suppressed, and a ductile product is developed. Such materials, known as metallic glasses, are described, for example, the book "Metallic Glasses", American Society for Metals, 1978.
In a typical process for making powder of these alloys, a liquid stream of the alloy is disrupted into particles by an impinging stream of gas (which may be nitrogen, 95% nitrogen, 5% hydrogen, etc.) or water. This process has been widely used for many years. The cooling rate of particles or alloy formed in this process was expected to be high, but it was not known whether the rate would be high enough to impart ductility to the particles.
Another method to make a foil is to form a flexible tape. Atomized nickel base alloy powder is suspended in an organic fluid containing wheat is, in effect, a glue. The resulting slurry is deposited on a plastic carrier strip by, for example, a doctor blade. In subsequent drying step, the organic carrier is removed, while the residual organic binder holds the particles together in a coherent strip. Removal of the binder causes problems as with powders and pastes.
The initial steps of a doctor blade, foil process are similar to those of the flexible tape process. A steel substrate is substituted for the plastic substrate. Nickel alloy powder is suspended in an organic vehicle to form a slurry. The organic binder which is necessary to the flexible tape process is omitted from the slurry.
Slurry is dispensed from a doctor blade arrangement to the steel substrate, passes through a drying system which removes the carrier vehicle, and then into a controlled atmosphere furnace, where the braze alloy is melted. In a separate operation, steel is removed from the braze alloy by chemical etching. Iron contamination can occur by solution of iron from the substrate.
In addition, borided foil is made by adding boron to the surface of a rolled ductile nickel alloy foil which contains all the constituents of the final brazing alloy except boron.
Boriding proceses which have been used include:
Gaseous (C.V.D.) deposition from diborane. PA1 Physical vapour deposition (P.V.D.). PA1 Solid pack boriding from boron carbide. PA1 Electroboronizing from a salt bath. PA1 Chemical deposition from molten borox.
Deposited boron is generally reacted with the substrate to form a nickel boride layer which is extremely well-bonded to the substrate.
Most of the boriding processes can be applied to any form of substrate, including wire and preformed parts. The step is not homogeneous, however.
U.S. Pat. No. 3,786,854 describes another method of making foils of these alloys by applying a paste of such alloys to a steel substrate, melting to form a homogeneous layer on the substrate, and subsequently etching away the steel to leave homogeneous alloy foil.
The resulting foil can be contaminated with iron by diffusion from the substrate. Etching of the steel from the substrate is difficult, particularly as the foil hs a cast structure, and is often very brittle.